Use of capacitance-type detection techniques for sensing level of material in a storage vessel has been widely proposed and is reasonably well understood in the art. In general, calibration in the field has been a time-consuming and laborious process requiring the efforts of a skilled or semi-skilled operator. There has been a need in the art for a system embodying facility for automatic on-demand calibration which does not require intervention by a skilled operator.
U.S. Pat. No. 4,499,766 discloses a system and probe for indicating the level of material in a vessel as a function of material capacitance. The disclosed system includes a resonant circuit having a capacitance probe adapted to be disposed in a vessel so as to be responsive to variations in capacitance as a function of material level. An oscillator has an output coupled to the resonant circuit and to a phase detector for detecting variations in phase angle as a function of probe capacitance. Level detection circuitry is responsive to an output of the phase detector, and to a reference signal indicative of a predetermined level of material, for indicating material level as a function of a difference between the reference signal and capacitance at the probe. Sensitivity of detection is adjustable for accommodating materials of differing dielectric constants. In the preferred embodiments disclosed in such application, a push-button automatic calibration circuit adjusts the resonance characteristics of the parallel resonant circuit, or adjusts the reference signal indicative of a predetermined reference material level.
U.S. Pat. No. 4,624,139 discloses a material level indicating system which includes a bridge circuit with a capacitance material level probe in one bridge arm. An adjacent bridge arm includes a plurality of fixed capacitors coupled to controlled electronic switches for selective connection into the bridge circuit. The bridge circuit is powered by an oscillator, and a differential amplifier is connected across the bridge circuit for detecting balance conditions at the bridge. A push-button automatic calibration circuit includes a digital counter having outputs connected to the electronic switches. A comparator is responsive to the differential amplifier for enabling operation of the counter during a calibration mode of operation for selectively connecting the fixed capacitors into the bridge circuit until a preselected balance condition, corresponding to a preselected reference material level, is obtained. Thereafter, the differential amplifier is responsive to variation of probe capacitance from the reference level to indicate material level.
Automatic calibration technology discussed in the preceding paragraphs has enjoyed substantial commercial acceptance and success in the material level control market. In systems heretofore marketed under the aforementioned U.S. Pat. No. 4,499,766, a pair of LED's are provided for indicating operating characteristics of the circuit. One LED is coupled to the detection circuit output for indicating whether material is detected adjacent to the probe. Another LED is coupled to the automatic calibration circuit for indicating that the system is calibrated when the LED is illuminated, and indicating loss of calibration or pendency of a calibration operation when the LED is extinguished. Loss of calibration can occur as a result of a build-up of electrically conductive material on the probe that electrically interconnects the probe element to the surrounding guard shield. Since the probe is now driven by the guard amplifier, this material bridge has the effect of desensitizing the probe. A deficiency in systems heretofore proposed lies in the fact that circuitry for detecting loss of system calibration is not responsive to system sensitivity. For example, in a system set to be highly sensitive for use in connection with materials of low dielectric constant, a relatively small amount of conductive material on the probe would be sufficient to impair system calibration. On the other hand, in systems set to lower sensitivity, a relatively greater amount of conductive material on the probe would be required before the system can no longer operate as calibrated. It is therefore desirable to tailor detection of loss of calibration to system sensitivity selected by the user, and a principal object of the invention is to provide a system of the described character in which this objective is achieved.